This post from the other thread created on the matter probably sheds the most light on this issue
http://www.cableforum.co.uk/board/34727126-post13.html
Wireless is never likely to perform quite as well as a wired connection. Local interference on the channel, overall signal strength, etc, will all have an impact on degradation. However you should still see a reasonable speed if all other aspects of the hardware and their drivers are working correctly, and you at least have "g" speed connetivity.